Plastics pipe

ABSTRACT

A plastics pipe which comprises an inner core and an outer removable skin layer bonded thereto, wherein the inner core and the outer removable skin layer comprise polymeric materials chosen to have matching Young&#39;s moduli, such that the Young&#39;s modulus of the skin layer is equal to or less than the Young&#39;s modulus of the inner core, and the adhesion of the skin layer to the inner core is sufficient to prevent substantial undesired relative movement between the skin layer and the core during installation, but insufficient to prevent the outer skin layer from being cleanly removed by peeling, at least at the ends of the pipe, and insufficient to cause a substantial reduction in the impact strength of the inner core.

This invention relates to plastic pipes and more particularly to a novelcomposite plastics pipe, a method for its manufacture, and a method formaking joints in such a pipe.

In the handling, installation and connection of plastics pipes, the pipesurface is easily damaged. In “no-dig” plastics pipe installationtechniques, for example, a tunnel is bored in the ground for the pipeand the pipe is then pushed or pulled through the tunnel into anexcavated hole where the next pipe joint is to be made. Installationtechniques such as pipe-bursting and slip lining can also place extremestress on the pipe surface.

Other modern pipe laying methods can also subject the pipe tosubstantial bending and tensile forces, both of which can result in adeterioration of the mechanical strength of the pipe. In addition, theuseful life of the pipe may be reduced by diffusible materials in theground, or by environment conditions, for example, exposure to directsunlight for long periods.

Of greatest concern, is that modern pipe laying methods can result inthe pipe becoming scratched and dirty. This is disadvantageous firstlyas the pipe material may be notch sensitive, in which case any scratchesmay cause greater damage to occur in the pipe during subsequent handlingor use. Secondly, dirt and/or oxidation on the pipe surface preventsuccessful welding. The main reason for failure of joints using anelectrofusion coupler is that the surface of the pipe is dirty or hasbecome oxidised. For this reason, until recently, the pipe ends alwayshave had to be cleaned and scraped before jointing, for exampLe, with ahand or mechanical scraper. In practice, the cleaning and scraping isoften uneven (the underside of the pipe in particular may be treatedless carefully) and the quality of the end result depends upon theprofessional skill of the installer.

In recent years there have been proposals to provide the pipe with anon-adherent skin layer which can be removed in order to permitjointing. Composite pipe constructions of this type are described, forexample, in JP3-24392, EP0474583, EP0604907, GB2323556, GB2300456, andWO93/00212. The entire disclosures of all these patents are incorporatedherein by reference for all purposes.

All of these prior art pipe constructions suffer from the disadvantagethat modern pipe laying techniques tend to cause wrinkling, rucking, orat least undesired movement of the non-adherent skin layer relative tothe core when the pipe is pushed through the ground. These proposalshave therefore not proved commercially acceptable.

More traditional proposals, wherein a protective skin layer is providedwhich is strongly adherent to the pipe, do not, of course, overcome theproblem of dirt and oxidation on the outer surface, since such skinlayers are very difficult to remove without elaborate equipment. Thepresence of a tightly adherent skin layer can also dramatically lowerthe impact strength of the plastics pipe.

The first appreciation that the above problems could be solved by usinga protective skin layer which is only lightly adherent to the core pipeoccurs in GB2297137 and GB2297138, the entire disclosures of which areincorporated herein by reference for all purposes.

In GB2297138, for example, there is provided a plastics pipe whichcomprises an inner core and an outer protective layer bonded thereto, inwhich the dimensions of the pipe and the protective layer are such thatthe ratio of the external diameter of the pipe to the thickness of theprotective layer is at least 70, preferably at least 100, and thecohesive strength of the outer protective layer, excluding any lines ofweakness, at least at the ends of the pipe, is greater than the strengthof the adhesive bond between the outer protective layer and the innercore. According to this specification, by a correct choice of thematerial of the skin layer and the extrusion conditions, it is possibleto provide a level of adhesion which still permits clean removal of theskin layer by peeling, whilst preventing rucking or wrinkling of theskin layer during installation and without substantially adverselyaffecting the mechanical properties of the pipe.

The composite pipe of UK patents GB2297137 and GB2297138 has beencommercially extremely successful, but it has been found that underspecific conditions of temperature and loading it is difficult toprovide a skin layer which has both the required toughness and limitedadhesion to the core pipe. Quality control of the base polymer materialof the skin layer, and control of the extrusion conditions duringmanufacture, need to be rigorously maintained if undesirable quantitiesof scrap are to be avoided. This substantially increases both rawmaterial and manufacturing costs.

It has now surprisingly been discovered that a substantial improvementin the impact strength of the composite pipe can be obtained by matchingcertain physical and mechanical properties of the polymeric materials ofthe core pipe and the skin layer.

In a first aspect, the invention provides a plastics pipe whichcomprises an inner core and an outer removable skin layer bondedthereto,

wherein the inner core and the outer removable skin layer comprisepolymeric materials chosen to have matching Young's moduli, such thatthe Young's modulus of the skin layer is equal to or less than theYoung's modulus of the inner core,

and the adhesion of the skin layer to the inner core is sufficient toprevent substantial undesired relative movement between the skin layerand the core during installation, but insufficient to prevent the outerskin layer from being cleanly removed by peeling, at least at the endsof the pipe, and insufficient to cause a substantial reduction in theimpact strength of the inner core.

In a further aspect, the invention provides a method for the productionof a plastics pipe comprising an inner core and an outer removable skinlayer bonded thereto, the inner core and the outer removable skin layercomprising polymeric materials chosen to have matched Young's moduli,such that the Young's modulus of the skin layer is equal to or less thanthe Young's modulus of the inner core, which method comprisesco-extruding molten polymeric materials forming the inner core and theouter removable skin layer from one or more extruder dies, bringing themolten polymeric materials together and allowing them to cool, suchthat, on cooling, the adhesion of the skin layer to the inner core issufficient to prevent substantial undesired relative movement betweenthe skin layer and the core during installation of the pipe, butinsufficient to prevent the skin layer from being cleanly removed bypeeling, at least at the ends of the pipe, and insufficient to cause asubstantial reduction in the impact strength of the inner core.

In another aspect the invention provides a method of making a joint to aplastics pipe according to the first aspect of the invention, or ofjoining two such plastics pipes, which comprises peeling the skin layerfrom the region or regions of the pipe to be joined, to expose a cleansurface suitable for electrofusion jointing, installing an electrofusioncoupler over the clean surface or surfaces of the pipe or pipes andactivating the electrofusion coupler to fuse the region or regions ofthe pipe or pipes thereto.

By “undesired relative movement” in this specification is meant movementor de-bonding of the skin layer relative to the core during directionaldrilling, pipe bursting, slip lining, or other conventional pipeinstallation procedures.

By selecting the polymeric materials of the inner core and the skinlayer on the basis of matching their Young's moduli, we have found thatit is possible to improve greatly the consistency of the resultant pipewithout sacrificing quality and performance. Thus the properties of thematerial of the skin layer no longer have to be a compromise betweenconflicting requirements.

By “matching Young's moduli” in the present specification is meant thatthe Young's moduli of the skin layer and the inner core are sufficientlyclose numerically that the composite pipe essentially behaves as asingle body under impact. Where the Young's modulus of the skin layer isless than the Young's modulus of the inner core, preferably the moduliof the inner core and the skin layer do not differ by more than 300 MPa,more preferably by not more than 150 MPa, and most preferably by notmore than 50 MPa, when measured, for example, by the method of DIN 53457-Z. Ideally the Young's moduli of the inner core and the skin layershould be identical, or nearly so.

Preferably the Young's modulus (in MPa) of the inner core is at least900, more preferably within the range from 950 to 1350, most preferablywithin the range 1000 to 1250. Preferably the Young's modulus of theskin layer (in MPa) is at least 800, more preferably within the range800 to 1350, and most preferably within the range 950 to 1250, whenmeasured, for example, by the method of DIN 53 457-Z.

The strength of the adhesive bond between the skin layer and the innercore is preferably at least 0.1 N/mm, more preferably at least 0.2 N/mm,when measured by a rolling drum peel test as described in Appendix 1.The adhesive bond between the skin layer and the inner core ispreferably less than 2.0 N/mm, more preferably less than 1.5 N/mm. Verygood results have been achieved using an adhesion between the skin layerand the inner core within the range of from 0.3 to 1.5 N/mm, whenmeasured by the above-mentioned rolling drum peel test.

Without wishing to be bound to any particular theory, it is believedthat the adhesion between the high molecular weight polymers of the skinlayer and the core is as a result of Van der Waals and/or diffusivebonding, or similar forces. Trhe adhesive properties of the innerbonding layer could be modified, for example, by the addition of anadhesion modifying agent such as a glycerol ester, as described inco-pending UK patent application no. (Agent's reference P071963GB).

It is likely that any adhesion between the skin layer and the inner corewill have some effect upon the impact strength of the plastics pipe, andit is presumably for this reason that prior art proposals (other thanGB2297137 and GB2297138) have always sought to avoid adhesion betweenthe skin layer and the core. Nevertheless, it has been found that bymatching the Young's moduli of the inner core and the skin layer andemploying limited adhesion, as discussed above, there can be produced acomposite plastics pipe having sufficient impact strength to meet therequirements of all available standards.

Preferably the polymeric materials of the inner core and the skin layerare matched such that the impact strength of the composite plastics pipeis at least 50%, preferably at least 75%, more preferably at least 90%of the impact strength of the inner core without the skin layer.

The inner core and the skin layer of the composite plastics pipe of thepresent invention can comprise any suitable thermoplastic polymericmaterials, consistent with the matching of their physical and mechanicalproperties. Suitable polymeric materials include, for example,olefinically-unsaturated polymers and co-polymers, for example,polyolefins such as polyethylene, polypropylene, polybutene andpolybutylene; ethylene and propylene co-polymers, for example,etlhylene-vinyl acetate polymers, and propylene-vinyl acetate polymers;halogenated-vinyl polymers such as vinyl chloride polymers andco-polymers; polyamides, for example, nylon 6, nylon 11 and nylon 66;polycarbonates; ABS polymers and ionomer polymers such as Surlyn (RTM).

The inner core of the pipe comprises a polymeric material chosen to becompatible with the particular application, and in particular with thefluid material to be conveyed by the pipe. For many applicationspolyethylene is the preferred material for the inner core. The grade ofpolyethylene chosen, that is to say, high density, medium density, lowdensity, or linear low density, will depend upon the particularapplication. Suitable grades of polyethylene for pressure pipe.applications preferably meet the requirements of at least one of prEN12201-1 (except clause 4.2.1 and the associated pigment or carbon blackrequirements if the PE material is unpigmented), prEN12201-2 (exceptclause 5.2 and the associated pigment or carbon black requirements ifthe PE material is unpigmented), prEN1555-1 (except clause 4.2.2 and theassociated pigment or carbon black requirements if the PE material isunpigmented) and prEN1555-2 (except clause 5.2 and the associatedpigment or carbon black requirements if the PE material is unpigmented).

Any suitable equivalent grade of polyethylene may, of course, also beused.

Preferably the inner core, where polyethylene is chosen, has an impactstrength of at least 300 joules, more preferably at least 400 joules andmost preferably at least 500 joules, when measured using the method ofEN1411:1996 at a temperature of −10° C. using a 90 mm diameter tup forimpacting the pipe.

The skin layer is formed from a polymeric material or a blend ofpolymeric materials having a Young's modulus matching that of the innercore. Preferred polymeric materials for the skin layer comprisepropylene homo- and co-polymers, propylene block co-polymers, andpropylene random co-polymers.

Preferably the skin layer has a notched Charpy impact strength of atleast 1 kJ/m², more preferably at least 2 kJ/m² and most preferably atleast 4 kJ/m², when measured using the method of ISO 179/16A at atemperature of −20° C.

A particularly preferred plastics pipe according to the presentinvention comprises an inner core of polyethylene and a skin layer of apropylene block co-polymer having matched Young's moduli.

Preferably the impact strength of a 90 mm outside diameter plastics pipehaving a polyethylene inner core and a polypropylene skin layer with anSDR of 17.0 is greater than 300 joules when measured using the method ofEN1411:1996 at a temperature of −10° C. using a 90 mm diameter tup forimpacting the pipe.

An advantage of the plastics pipes of the present invention is that thenormal UV stabiliser and colorant package need not be included in theplastics material of the inner core, provided that sufficient quantitiesof these materials are included in the skin layer. This enables theinner core to comprise a natural polymeric material, free orsubstantially free from additives which add to the cost of the corematerial and which, in certain circumstances, may impair the mechanicalor physical properties of the core material. Alternatively, stabiliserscan be included in the core material, but the outer protective skinlayer can be coloured to indicate the fluid being transported within thepipe.

Suitable stabiliser or ultra-violet blocking additives include, forexample, titanium dioxide, carbon black, and other fillers. Whilstcarbon black is an excellent UV stabiliser and reinforcing filler,buried pipes are frequently colour coded and its use in the outerprotective layer is therefore not possible for many applications.Titanium dioxide is, therefore, the preferred filler and UV stabilisersince this is also compatible with many colcorant packages. Other fillermaterials such as chalk and talc, may also be used. The preferred fillerparticle size is dependent on the filler being used, but for titaniumdioxide, for example, the average particle size range is preferable from0.003 to 0.025 microns.

The skin layer and the inner core can, of course, each comprise morethan one layer of polymeric material, although in practice this is notusually necessary.

The relative thickness of the skin layer and the dimensions of the pipehave also been found to affect the impact resistance of the pipe. Thisis discussed in GB 2297138. Preferably the skin layer has a thickness ofgreater than 0.1 mm, more preferably greater than 0.2 mm, and mostpreferably within the range of from about 0.3 mm to 2.0 mm.

The dimensions of the pipe and the protective layer are preferably suchthat the ratio of the external diameter of the pipe to the thickness ofthe skin layer is at least 70, more preferably at least 100, mostpreferably in the range 150 to 800. From this it can be seen that it ispossible to use a thicker skin layer on a pipe of greater diameter.

When stripping the skin layer from the pipe, it is important that noresidue or holidays should be left on the pipe surface that couldinterfere with the electrofusion jointing process. Thus conventionaladhesives and skin layers that are prone to tearing or fragmentationshould be avoided. In general the force required to rupture the skinlayer should be greater than the force required to peel the skin layerfrom the inner core.

By “a clean surface” in this specification is meant a pipe surface thatcan be subjected to electrofusion jointing without further preparationor treatment. Such surfaces should meet the requirements of one or moreof pr EN12201 part 3, pr EN1555 part 3 and WIS 04-32-14.

The composite plastics pipe of the present invention is preferablyproduced by co-extrusion, wherein the polymeric materials are broughttogether in the pressure area of the die and exit as a single extrudate.For example, the die may be connected to one, two, or more extruders andfed with separate streams of molten material. Alternatively, the die maybe provided with concentric die outlets fed with separate streams ofmolten polymeric materials which are to form the inner core and the skinlayer. In this case, the extrudates, on leaving the extruder dieoutlets, can be brought into contact with each other in a sizing diewhich simultaneously adjusts the outer diatmeter of the pipe.

In a further alternative, the inner core extrudate may be passed througha sizing die before applying the skin layer. In this case it may benecessary to re-heat or flame-brush the surface of the inner coreextrudate to create a surface ready to receive the skin layer. Becauseof the difficulty of maintaining a consistent adhesion between the innercore and the skin layer, this method is not presently preferred.

The invention is illustrated by the following Example:

EXAMPLE

A polyethylene core pipe of nomimal outer diameter 90mm was co-extrudedwith a propylene random copolymer skin layer. The experiment wasrepeated replacing the propylene random copolymer with a propylene blockcopolymer.

Skin adhesion was measured using a rolling drum peel test as describedin Appendix 1.

The skin layers of the resultant pipes could be peeled readily using asimple hand tool, exposing a clean surface of the core pipe.Electrofusion jointing tests gave very good results in conformance withprEN12201 part 3, prEN1555 part 3 and WIS 04-32-14.

The impact strength cuf the pipes was measured at −10° C., with andwithout the skin layer, using the method of EN 1411:1996. In furtherexperiments the pipes were notched at 90° to the point of impact priorto testing to simulate service conditions. The results are given inTable 1. TABLE 1 PIPE 1 2 Ø(MM)-SDR 90-17 90-17 CORE resin PolyethylenePolyethylene Young's modulus MPa 1150 1150     SKIN resin PropylenePropylene random block copolymer copolymer Thickness (mm) 0.5-1.0 0.67Young's modulus MPa 900 1100     SKIN ADHESION Min(N/mm) 0.3 1.08 Mean(N/mm) — 1.55 Max (N/mm) 0.8 1.96 IMPACT at −10° C. Un-Notched (J)103-324 >588     No skin, un-notched (J) 487-588 Notched to 0.2 mm 103333*    depth (J) No Skin, notched (J) 387*Passed if the skin breaks but the core remains intact.

The results show that by matching the Young's moduli of a propylenecopolymer skin layer to a polyethylene core, the impact strength of thepipe can be made comparable to a similar pipe without a skin layer,whilst the peelability of the pipe is retained.

APPENDIX 1

DETERMINATION OF THE ADHESION STRENGTH OF PIPE SKIN—CORE PEEL

Apparatus

A tensile testing machine accurate to grade A of BS5214 Part 1: 1975 orgrade 1 of BS1610: Part 1: 1985, for example, a Lloyds tensile testmachine, using a 100N load cell.

Test Specimens

Two test pieces are cut one from each end of the sample pipe, 25 mm+/−2mm wide, the two sample rings of pipe are trimmed around thecircumference to remove the jagged edge. The pipe is marked along topdead centre (TDC) of the extrusion line (if known).

The two ring specimens are marked with an indelible marker at quarterlypoints around the circumference beginning at TDC (if known), asillustrated in FIG. 1.

Procedure

Cut through the skin along mark at TDC & prise edge of skin from pipe,peel skin off to 30-40 mm length, feed peeled skin through the jig asshown & clamp in upper jaws.

Mount the test piece in the jig as shown in FIGS. 2 and 2 a.

The skin is then peeled from the pipe at a separation rate of 100 mm/minand a trace recorded of load versus time.

The average value of the load required to peel the skin sample iscalculated (Newtons), and divided by the true width of the peel sampleto obtain the test result (Newtons/millimetre)

The average of the 10 peak load values recorded is calculated (Newtons),and divided by the true width of the peel sample to obtain the testresult (Newtons/millimetre).

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent, or similar purpose, unless expresslystated otherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of any foregoingembodiments. The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

1. A plastics pipe which comprises an inner core and an outer removableskin layer bonded thereto, wherein the inner core an the outer removableskin layer comprise polymeric materials chosen to have matching Young'smoduli, such that the Young's modulus of the skin layer is equal to orless than the Young's modulus of the inner core, and the adhesion of theskin layer to the inner core is sufficient to prevent substantialundesired relative movement between the skin layer and the core duringinstallation, but insufficient to prevent the outer skin layer frombeing cleanly removed by peeling, at least at the ends of the pipe, andinsufficient to cause a substantial reduction in the impact strength ofthe inner core.
 2. A plastics pipe according to claim 1, wherein whenthe Young's modulus of the skin layer is less than the Young's modulusof the inner core, the moduli of the inner core and the skin layer donot differ by more than 150 MPa, when measured by the method of DIN 53457-Z.
 3. A plastics pipe according to claim 1, wherein the moduli ofthe inner core and the skin layer do not differ by more than 50 MPa,when measured by the method of DIN 53 457-Z.
 4. A plastics pipeaccording to claim 1, wherein the Young's modulus of the inner core iswithin the range from 950 to 1350 MPa.
 5. A plastics pipe according toclaim 1, wherein the Young's modulus of the skin layer is within therange from 800 to 1350 MPa.
 6. A plastics pipe according to claim 1,wherein the strength of the adhesive bond between the skin layer and theinner core is from 0.3 N/mm to 1.5 N/mm, when measured by a rolling drumpeel test as described in Appendix
 1. 7. A plastics pipe according toclaim 1, in which the strength of the adhesive bond between the skinlayer and the inner core is such that the impact strength of thecomposite pipe is at least 75% of the impact strength of the inner corewithout the skin layer.
 8. A plastics pipe according to claim 1, inwhich the inner core comprises polyethylene.
 9. A plastics pipeaccording to claim 1, wherein the skin layer comprises a propylene homo-or co-polymer, or a propylene block co-polymer.
 10. A plastics pipeaccording to claim 9, wherein the skin layer comprises a propylene blockco-polymer.
 11. A plastics pipe according to claim 1, in which the innercore comprises polyethylene and the skin layer comprises a propyleneco-polymer and wherein the impact strength of the pipe is greater than300 joules, when measured using the method of EN1411:1996 at atemperature of −10° C. using a 90mm tup for impacting the pipe.
 12. Aplastics pipe according to claim 1, wherein the skin layer has athickness within the range of from 0.3 mm to 2.0 mm.
 13. A plastics pipeaccording to claim 1, wherein the ratio of the external diameter of thepipe to the thickness of the skin layer is from 150 to
 800. 14. A methodfor the production of a plastics pipe comprising an inner core and anouter removable skin layer bonded thereto, the inner core and the outerremovable skin layer comprising polymeric materials chosen to havematched Young's moduli, such that the Young's modulus of the skin layeris equal to or less than the Young's modulus of the inner core, whichmethod comprises co-extruding molten polymeric materials forming theinner core and the outer removable skin layer from one or more extruderdies, bringing the molten polymeric materials together and allowing themto cool, such that, on cooling, the adhesion of the skin layer to theinner core is sufficient to prevent substantial undesired relativemovement between the skin layer and the core during installation of thepipe, but insufficient to prevent the skin layer from being cleanlyremoved by peeling, at least at thee ends of the pipe, and insufficientto cause a substantial reduction in the impact strength of the innercore.
 15. A method according to claim 14, wherein the polymericmaterials of the inner core and the outer removable skin layer areextruded simultaneously and brought together whilst still hot. 16.(canceled)
 17. A method of making a joint to a plastics pipe accordingto claim 1, or of joining two such plastics pipes, which comprisespeeling the skin layer from the region or regions of the pipe to bejoined, to expose a clean surface suitable for electrofusion jointing,installing an electrofusion coupler over the clean surface or surfacesof the pipe or pipes and activating the electrofusion coupler to fusethe region or regions of the pipe or pipes thereto.